With the continuous development of the national economy, electricity consumptions of traditional heavy loads continue to grow. If the loads are far away from the power center, the electricity is transmitted between regional power systems with a high power. Interconnection ways of the power systems have been widely applied, such as long distance transmission lines of UHVDC (ultra-high voltage direct current), long distance transmission lines of UHVAC (ultra-high voltage alternating current) and back to back DC (direct current) transmission systems.
It may be seen from field operations that, there are weak grids of near direct current areas at receiving ends in DC interconnection systems. In the near direct current areas, voltage levels are determined by power flows and electrodes in the area are closely associated. When DC transmission ways between the regional power systems change, active power flows in the near areas will change hugely; in addition, because converter stations are usually equipped with large-capacity filter capacitors, large reactive power excitations are generated when switching the capacitors. Thus, the voltages in convertor stations of the near direct current areas may fluctuate drastically, which is very unfavorable for a safe operation of the power system.
Currently, methods for participating in regional automatic voltage controls of the near direct current areas are relatively limited, which mainly include a method of using reactive powers of adjacent power plants and a method of switching capacitors and reactors in the convertor station. In the convertor station, only local voltage is controlled. Operation data shows that when the DC transmission power changes, the filter capacitors in the convertor station operate frequently, such that the system voltage may fluctuate drastically. The traditional voltage controls are mainly based on a system state at a single-time section, which cannot effectively respond to the mutual influences of varieties of reactive power compensation devices in the areas in a long process.